THE MUSCLES OF THE ANAL REGION 



425 



deeply between the Levator ani and Obturator internus into a space known as the 

 ischiorectal fossa. 



The Deep Fascia. The deep fascia forms the lining of the ischiorectal fossa; 

 it comprises the anal fascia, and the portion of obturator fascia below the origin 

 of Levator ani. 



Ischiorectal Fossa (fossa ischiorectalis) (Fig. 405). The fossa is somewhat pris- 

 matic in shape, with its base directed to the surface of the perineum, and its apex 

 at the line of meeting of the obturator and anal fasciae. It is bounded medially 

 by the Sphincter ani externus and the anal fascia; laterally, by the tuberosity of 

 the ischium and the obturator fascia; anteriorly, by the fascia of Colles covering 

 the Transversus perinsei superficialis, and by the inferior fascia of the urogenital 

 diaphragm; posteriorly, by the Glutseus maximus and the sacrotuberous ligament. 

 Crossing the space transversely are the inferior hemorrhoidal vessels and nerves; 



FIG. 405. The perineum. The integument and superficial layer of superficial fascia reflected. 



at the back part are the perineal and perforating cutaneous branches of the 

 pudendal plexus; while from the forepart the posterior scrota! (or labial) vessels 

 and nerves emerge. The internal pudendal vessels and pudendal nerve lie in 

 Alcock's canal on the lateral wall. The fossa is filled with fatty tissue across 

 which numerous fibrous bands extend from side to side. 



The Corrugator Cutis Ani. Around the anus is a thin stratum of involuntary 

 muscular fiber, which radiates from the orifice. Medially the fibers fade off into 

 the submucous tissue, while laterally they blend with the true skin. By its contrac- 

 tion it raises the skin into ridges around the margin of the anus. 



The Sphincter ani externus (External sphincter ani) (Fig 405) is a flat plane 

 of muscular fibers, elliptical in shape and intimately adherent to the integument 

 surrounding the margin of the anus. It measures about 8 to 10 cm. in length, from 

 its anterior to its posterior extremity, and is about 2.5 cm. broad opposite the 

 anus. It consists of two strata, superficial and deep. The superficial, constituting 



